March is best known for her role as Assistant District Attorney Alexandra Cabot on the NBC crime legal drama series, Law & Order: Special Victims Unit, a role she played from 2000 to 2003. She departed in the season 5 episode "Loss", when Cabot is shot and placed in witness protection. She returned to the series for a six-episode arc in season 10 and then continued as a main character in season 11. After her departure near the end of season 11, she returned again in season 13 as a recurring character.

March reprised her role as Cabot, now the Bureau Chief ADA of the homicide bureau, on the short-lived NBC courtroom drama Conviction, which debuted in spring 2006. The show, which was part of the Law & Order universe, only lasted one 13-episode season before being cancelled.[9]

March is an advocate for women's rights and a supporter of Planned Parenthood. In 1938, her great-grandmother, Ruby Webster March, founded the West Texas Mother's Health Center, which later became part of Planned Parenthood of West Texas.[13]

March wrote an essay, published in June 2016, that detailed her experience with breast augmentation surgery in 2014, and the subsequent removal of the implants due to infection.[14]